Permeability evolution and proppant compaction in artificial fractures on Green River Shale

Through laboratory experiments with artificial propped fractures in Green River Shale, this paper compares the evolution of permeability for native CH4 and with those of sorbing CO₂, slightly sorbing N₂ and non-sorbing He, as a function of pore pressure. The findings from these experiments help to understand proppant embedment and fracture diagenesis in shales. Experiments were conducted on linch diameter, 2inch long split cylindrical samples sandwiched with proppant at a constant confining stress of 20 MPa and with varied pore pressure - increases in pore pressure represent concomitant decreases in effective stress. Permeability and sorption characteristics are measured by pulse transient methods. To explore the effect of swelling and embedment on fracture surface geometry, we measure the evolution of transport characteristics for different proppant geometries (single layer vs. multi-layer), gas saturation, and sample variance. In order to simulate both production and enhanced gas recovery processes, both injection and depletion cases are investigated. For both strongly- (CO₂, CH₄) and slightly-adsorptive gases (N₂) the permeability first decreases when gas pressure increases because of swelling. It then increases beyond the Langmuir threshold due to the over-riding influence of effective stresses. Due to its highest adsorptive affinity, CO₂ returns the lowest permeability among these three gas permeants. Compared to the case of a mono-layer propped sample, the sample with four layers exhibits less swelling as implied by its elevated k/ko ratio. Interestingly the duration of gas exposure and saturation tested here which is up to ∼20hrs does not have a significant influence on permeability for either adsorptive or non-adsorptive gases. Permeabilities recovered from both injection and depletion cycles generally overlap each other and are repeatable with little hysteresis. This suggests the dominant role of reversible swelling over irreversible embedment. Permeability variance between different samples is of the order of-1.5-2 times but with repeatable trends and order of magnitude parity. Gas permeant composition and related swelling effects exert important influences on the permeability evolution of shales under nominally in situ conditions.